But when it comes to defending those values they believe there is more that we – the Brits – should be prepared to do. “Hey, you guys,” was the message I got, “we hope you don’t go all soft on us now.” They look at Vladimir Putin, and they are alarmed. They see him carving up Ukraine; they know that he is causing increasing anxiety in Russia’s near abroad, especially among the Baltic states. And this, they say, is no time to be cutting defence expenditure.

“You are our number one ally,” as one Congressman put it to me, “and we need you.” And, of course, I protested strongly: that we are there, that we always have been, and – at least as long as David Cameron is Prime Minister – that we always will be there. It was the Prime Minister who led the way at the recent Cardiff summit in committing Nato countries to spend 2 per cent of GDP on defence.

It was David Cameron – and not the White House – who was in the lead in calling for intervention in Syria in 2013; just as it was the Prime Minister who was instrumental in ending the massacre in Benghazi: the right thing to do, even if there has been an unhappy sequel. British Armed Forces have done a superb job in Sierra Leone, and generally in the struggle against ebola. You will find British troops deployed in dozens of countries around the world, and the Royal Navy on patrol everywhere from the horn of Africa to the Gulf to the Strait of Malacca to the Falklands to the Mediterranean.

We are still the fourth biggest military power in the world, we have the best special forces, and we have just invested £6 billion in two colossal new aircraft carriers. We are the one ally that has been with America – in spite of all our doubts and public protest – throughout the long and bitter engagements in Iraq and Afghanistan, and whatever else you may say about this country, you could not accuse us of lacking a historic martial spirit. It is not necessarily something to brag about, but it is nonetheless a fact that of the roughly 200 countries in the world today, Britain has at one time or other invaded or conquered 178 of them. The only people to escape are places like Luxembourg.

There is no other country that comes close to that record of belligerence; not the Americans, not the French, not even the Romans. These days, of course, we have not the slightest intention of invading or conquering anyone – not least after the unhappy experience of the Iraq war. All we want is to do our very considerable best to help keep the world safe; and our American friends are, of course, right to think that our defence budgets – like those around Europe – are under strain.

We face the increasing “juridification” of conflicts, with the MoD coughing up untold millions in ludicrous “compensation” to the many hundreds of jihadis who are using UK taxpayers’ money to sue the British Army for alleged breaches of their human rights.

The MoD must shoulder ever-growing costs in manpower, and defence budgets are by no means protected, or “ring-fenced”, like those of the NHS. All these problems are trivial, however, in comparison with the risk of a Labour government, and one led by the most left-wing leader since Michael Foot. For all his faults, Tony Blair correctly took the view that Britain is a great power, a moral force for good in the world, and one that must be ultimately capable of protecting those values by force. Ed Miliband has junked that tenet, along with the rest of Blairism.

It is now clear that if he were to govern at all – a prospect that seems less and less likely, but which cannot be dismissed – he would be kept in office by the votes of the SNP. Nicola Sturgeon, the leader of the Scottish National Party, has made it plain that her support is entirely conditional on one thing: that Labour gets rid of Trident. There would be no modernisation of our nuclear deterrent in 2016. Under Labour and the SNP, Britain would be denuding itself of its most important weapon; at the very moment when Putin is increasing his defence spending by 35 per cent, and building huge new drones capable of long-range bombing. How are we supposed, in those circumstances, to help the Americans face him down?

If a Labour-SNP coalition were to junk Trident, Britain would be vulnerable to nuclear blackmail; but it is worse than that. We would suffer a public and visible diminution of global authority; we would be sending a signal that we no longer wished to be taken seriously; that we were perfectly happy to abandon our seat on the UN Security Council to some suit from Brussels; that we were becoming a kind of military capon. Yes, the nukes are expensive – but so is all defence spending, these days.

The only way to fund the forces we need is to have a government that understands business, and produces sustained economic growth – and that cannot be Miliband. Our Armed Forces are not a luxury. They are indispensable to our lives. I remember how they rescued the position in the Olympic and Paralympic games – cheerfully helping with the security at the last minute. I have seen them stop London houses from being flooded, quickly and efficiently building sandbag fortifications.

But their role is much more important than that. As our American friends instinctively understand, it is the existence of strong and well-resourced British Armed Forces that gives this country the ability to express and affirm our values overseas: of freedom, democracy, tolerance, pluralism. David Cameron gets that. Ed Miliband would put it all at risk, and in the process he would make Britain weaker and less safe.